A vessel such as a tanker usually stores water called ballast water in a ballast tank provided on the vessel in order to balance the vessel under way while travelling toward another destination after unloading freight such as crude petroleum. Ballast water is normally charged at an unloading port and discharged at a loading port. If these ports are located at different places, microbes such as plankton and bacteria in the ballast water travel all over the world. When ballast water is discharged at a loading port in a sea area different from that of an unloading port, microbes in the other sea area are released at the loading port and may destroy the ecosystem in the sea area. In order to prevent such ballast water from destroying the marine environment, the International Maritime Organization (IMO) has entered into the International Convention for the Control and Management of Ships' Ballast Water and Sediments and has set a ballast water discharge standard to restrict the microbe content in ballast water discharged from ships.
This ballast water discharge standard sets the discharge limits for plankton according to sizes, specifically, at most 10 individuals/m3 for plankton of 50 μm or more and at most 10 individuals/m3 for plankton of 10 to 50 μm. The ballast water discharge standard also sets the discharge limits for bacteria, such as less than 250 cfu/100 ml for coliform.
Ballast water to be stored in a ballast tank is thus required to be treated by killing microbes and the like in the ballast water.
An ultraviolet treatment method has been known as a measure for treating ballast water by killing microbes and the like therein. According to this method, treatment target water drawn as ballast water is irradiated with ultraviolet (see Patent Literature 1, for example).
At an early stage of ballast water drawing, water around a vessel is shaken and sand or mud at the bottom of the sea is raised. As a result, in the filter, the efficiency of filtering of the treatment target water may be decreased by the influence of the rising sand or mud, or the filter may become clogged early. In this case, ballast water may flow into a ballast tank while treatment targets therein are killed insufficiently due to insufficient ultraviolet illuminance.